My current computer has always run unusually hot no matter what steps I have taken such as: lots of fans—including four 120mms; thermal grease—both generic and Arctic Silver V; placement of the system—ie on top of a desk hutch in open air; etc.

My case came with an 80mm fan hole in the center of the side panel. I tried putting a fan there both blowing in and blowing out, but neither had much effect. I tried putting a 120mm fan just over the CPU, but again little effect. I even tried putting four 120mm fans on the side of the case with no effect (other than lots of noise). I also tried using a new PSU with a 120mm fan built in right over the CPU; again little effect. I cut a 120mm hole in the top of the case and put a fan blowing out there and that seems to have had the most effect of everything that I have tried. So now I am looking for information on fan placement and direction.

Unfortunately I am having trouble finding information on optimizing the actual placement and direction of fans in a computer case. Where can I find practical information on what fans (eg size) to put on what parts of the case, and in which direction to have them blowing. That is, not-too-technical thermodynamic/aerodynamic/fluid-dynamic tips for air-cooling a computer.

Thanks a lot.

Edit - To address some questions:

The CPU used to run between 45-60°C (I actually had to set the BIOS CPU throttling to 70°C in the summer).

I am aware of the whole airflow concept for computers (in one side, out the other). However, none of the configurations I had tried had given good results; I have even tried taking all of the panels off of the case and leaving the motherboard in free, open air without success. Which is why I asked for specific help.

I do clean out the dust and oil the fans now and then.

My case is the main problem since it has specific fan holes built in, which reduces the places that I can put fans (at least without cutting it up). For example, the rear has a metal panel that has an 80mm and a 60mm fan hole, that’s all. There is a double-80mm fan hole at the front-bottom of the metal panel of the case, but it is covered by the plastic bezel with the power and reset buttons, so it is completely blocked off.

I have already maximized the space inside the case by using rounded cables, running wires along the edges, and putting my expansion cards and drives in a configuration that leaves as much central space as possible.

How much is unusual hot? Also, if you place multiple fans in a case, always check the direction, you need an air-flow with in the case (f.e. not one fan blowing directly onto/against the cpu fan).
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BobbyOct 27 '09 at 13:46

Here are a couple of more articles I have found on the subject, including discussion of positive and negative air pressure and hot-air recycling: [1][2]
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SynetechJul 7 '12 at 20:03

7 Answers
7

It was our assumption that the tests with ALL the fans in operation would produce the best results but it didn’t. Time to idle represents how effectively the configuration removes heat from the PC case. The shorter the time the better. CPU peak and idle as well as System peak and idle are easy to interpret. We would like to think that System temperature represents an average of how cool every component in a PC is.

The top and rear exhaust produced the best CPU and System results but nearly placed last for time to idle. We tested three times for this result as we didn’t believe the first two.

A single rear exhaust fan produces the best results overall. This flushes the theory of more is better right out the door.

A top only or top and front combination places in the middle of the pack for CPU and System peak cooling BUT does whisk away the heat in a very short amount of time.

Even with no cooling fans besides the heatink…heatsink size and type of fan can deliver good results.

So there you have it. A few theories dashed upon the rock perhaps. At least for this type of PC case and components. What should be taken away from this? Quite simply that more may not necessarily be better but, for us enthusiasts, more may be cool…for looks.

Well that’s pretty good. I was hoping for an article that was more scientifically based (the ideas, not the presentation), rather than experimental (that article seems to be more trial-and-error based). However, it is pretty good and the only useful one I have seen. Thanks.
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SynetechOct 28 '09 at 19:10

What you want to do is move air through the PC. You want the case to be a bit like a wind tunnel. The best tactic is to have one intake and one outtake. For low noise I recommend a slow moving 12cm fan at each point. Since warm air rises it's also best if your outtake is somewhere high and your intake somewhere low.

I happen to have an inexpensive (50 euro) case with an intake 12cm fan at the bottom front and an outtake 12cm fan at the top back with both fans running at half speed. The amount of cooling this simple setup provides to all the components is considerable.

A note about intake fans though; they suck in lots of dust, so you'd be well advised to have some sort of air filter (a piece of stocking could work, if you can't get an actual filter). Also you could run the intake fan just a bit faster than the outtake fan to create pressure inside the case that will keep dust from accumulating on all the little holes and cracks throughout the case.

More fans is not usually the answer. Likely the problem you are having is not a good flow, conventional wisdom is to suck air in the front and out the back. It is important to have even amount coming in and going out or you will end up with a bunch of loud fans that can not push air into the case.

Also to make sure the flow is unrestricted you should work to make sure there is an open path for the air to move through the case. With todays longer video cards I have noticed it is easy to have a video card that nearly touches the harddrive which basically cuts the case into two sections and blocks flow.

If you are looking for specific placement tips, I have a case right now that is exceptionally good at cooling the processor. The case is designed such that the side has a hole cut directly above the CPU. The case also has a shield that partially encloses the top of the CPU fan, this makes an easy path to pull cool outside air directly onto the CPU (my setup) or if you prefer push the hot air from the CPU outside of the case (I found this less effective).

Finally the obvious but sometimes forgotten thing is that heatsinks fill up with dust overtime, it is possible that your heatsinks just need cleaned so you should use some compressed air and blow out the fins of the heatsink. A tip to remember when using compressed air is to hold the fans in place with a finger, letting a fan spin freely while your blowing air through it can actually damage the fan.

Good point, especially with modern cards. Thanks for the link to the great article. The point about dust is good to consider too. It's a little odd that the positive-pressure diagram they show indicate the air being sucked into the case from the top even though it is generally accepted that it is more efficient to suck air in from the bottom and exhaust it out the top. I suppose you could put the filtered intake fan on the bottom to combine the best of both designs (maybe add some "high-heels" to the case's feet to give it i a little more clearance so that the fan has enough air to suck in).
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SynetechJun 27 '12 at 14:20

The "reference" is overly focused on the GPU creating or using recycled air. You should also be focusing on the ATX PSU with a 120mm fan. That style of PSU will recycle about 20% to 40% of its intake back into the case. Plus this recycled air has been heated up by the PSU. IOW a cooler PC is more likely when you avoid the PSU w/120mm fan, and use a PSU w/a traditional 80mm exhaust fan.
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sawdustAug 22 '12 at 22:09

Test airflow by holding a thread or small piece of paper in your case, but avoid getting into a fan!
I started at the power supply and intake fan and spotted a bad (graphic adapter) bypass flow.

You will see quickly if two fans fight airflow direction, often the CPU fan and power supply fan.

Since you want the CPU temperature down, make it getting outside air directly. I hand-made a short 12cm pipe from a hole in the middle of one side of my case to the CPU in order to force the CPU fan taking only fresh air.

I've done the positive air pressure system with the outs of case filters and a lot of trial and error with the help of temp monitoring. My case had many vents which I duct taped to get pressured system and air moving up (heat rises) from all sides except for the PSU and the 2 GPUs that have their own fans exiting to the back. I duct tape the bottom and the vents not wanting heated air to be sucked back into system.

No bad. That article is mostly the same as the one that aeinstein posted, but with some extra useful tips. Also, your idea about sealing holes is good; I did the same thing to fix a handheld vacuum a while back.
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SynetechMay 19 '13 at 19:36

buy a big case with lots of fan space, i tryed turning differant fans around 180 and found improvement. Got a coolmaster case with 3 140's and a 120 pushing in. Cpu is got 140 x 280 radiator with 2 120's going out of top . I rarely see temps over 45 C and the thing has 2 video cards, two hard drives, 16 gigs ram and 1000 watt power supply. It runs cooler with more fans pushing in and magnetic filter covers on "in" fan's really helps with dust